Simplify code of Add/Remove trailing slash and fix bug

[im-kulikov]

• simplify code (more informative / understanding)
• fix trailing slash bug
• assert collides with imported package name (in tests)
• fix unhandled errors

---

cc @vishr / @alexaandru

[codecov]

Codecov Report

Merging #1275 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1275   +/-   ##
=======================================
  Coverage   81.78%   81.78%           
=======================================
  Files          26       26           
  Lines        1933     1933           
=======================================
  Hits         1581     1581           
  Misses        243      243           
  Partials      109      109

Impacted Files	Coverage Δ
middleware/slash.go	91.48% <100%> (ø)	⬆️

---

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[im-kulikov]

and this PR fix next error:

package main

import (
	"fmt"
	"net/http"
	"net/http/httputil"
)

func check(err error) {
	if err != nil {
		panic(err)
	}
}

func main() {
	cli := new(http.Client)

	req, err := http.NewRequest(http.MethodConnect, "http://localhost:8080", nil)
	check(err)

	data, err := httputil.DumpRequest(req, true)
	check(err)

	fmt.Println(string(data))

	res, err := cli.Do(req)
	check(err)

	data, err = httputil.DumpResponse(res, true)
	check(err)

	fmt.Println(string(data))
}

2019/02/05 03:19:25 http: panic serving [::1]:56897: runtime error: index out of range
goroutine 84 [running]:
net/http.(*conn).serve.func1(0xc000166000)
	/usr/local/Cellar/go/1.11.5/libexec/src/net/http/server.go:1746 +0xd0
panic(0x18783c0, 0x20938c0)
	/usr/local/Cellar/go/1.11.5/libexec/src/runtime/panic.go:513 +0x1b9
github.com/labstack/echo/v4/middleware.AddTrailingSlashWithConfig.func1.1(0x1a771a0, 0xc0001ac070, 0xc0001ac070, 0xc000075d38)
	/Users/kulikov/.golang/pkg/mod/github.com/labstack/echo/[email protected]/middleware/slash.go:52 +0x33b
github.com/labstack/echo/v4.(*Echo).ServeHTTP(0xc0003948c0, 0x1a676c0, 0xc000392000, 0xc00014e300)
	/Users/kulikov/.golang/pkg/mod/github.com/labstack/echo/[email protected]/echo.go:593 +0x220
net/http.serverHandler.ServeHTTP(0xc0000b5ee0, 0x1a676c0, 0xc000392000, 0xc00014e300)
	/usr/local/Cellar/go/1.11.5/libexec/src/net/http/server.go:2741 +0xab
net/http.(*conn).serve(0xc000166000, 0x1a685c0, 0xc00013c400)
	/usr/local/Cellar/go/1.11.5/libexec/src/net/http/server.go:1847 +0x646
created by net/http.(*Server).Serve
	/usr/local/Cellar/go/1.11.5/libexec/src/net/http/server.go:2851 +0x2f5

[im-kulikov]

I corrected the description. In addition to simplifying and understanding the code, this PR fixes bug in the runtime.

Thoughts @vishr @alexaandru?

[alexaandru]

Nice cleanup! 👍